It is common practice to coat sections of new pipelines off-site in a pipe manufacturing operation or at a site other than in-line on the pipeline, to protect the pipeline from corrosion or mechanical damage. It is important for the long term protection of the pipeline that all sections of the pipeline have a viable coating.
Pipeline coatings invariably became damaged or defective, as a result of damage during installation, movement of rocks or other abrasive materials in the ground e.g. due to settling of soil or freeze/thaw cycles in certain areas, or aging of the coatings.
A wide variety of portable field-applied coatings are in use to recoat previously coated, and partially protected, aged metallic substrates e.g. in service steel pipeline fittings and assemblies and steel lined pipe. Such aged substrates are normally coated with e.g. mill-applied fusion bonded epoxy powder, extruded bitumen/polyethylene tape wraps, heat-shrink sleeves, two-part liquid epoxies applied by brush, by roller or by airless spray equipment; and modified liquid urethanes which are also applied either by brush, by roller or by airless spray equipment.
While each of these field-applied coatings is useful in certain applications, there are compatibility and performance limitations on both new and aged metallic substrates. Moreover, in situ coating repairs are difficult or impossible to accomplish in a manner that re-coats the pipeline with the coating originally applied to the pipe in a plant site.
Fusion bonded epoxy powders generally exhibit excellent adhesion and excellent cathodic disbandment properties. However, these coatings tend to be brittle and prone to handling damage and to display high moisture permeation. The coatings are useful as a new coating on metallic substrates, but generally are not chemically compatible with other coatings such as extruded plastics and plastic film. This excludes them from use with such dissimilar coated surfaces. For instance, fusion bonded epoxy powders, by themselves, are not compatible or reliable coatings with two or three layer, mill-coated composites as found on new steel line pipe.
Thermoset coatings, which include two-part liquid epoxy and two-part liquid urethanes, generally exhibit excellent adhesion and cathodic disbandment properties. However, these coatings also tend to be brittle and prone to handling damage and to display high moisture permeation. Thermoset coatings work well as either a new or recoat coating when the already-applied coating is also a thermoset material or a fusion bonded epoxy powder. However, they are not chemically compatible with other coatings such as extruded plastics and plastic films, and tend not be usable with such dissimilar coated surfaces. Thermoset coatings, by themselves, generally are not reliable with two or three layer mill-coated composites as found on new steel line pipe.
Tape wraps and heat-shrink sleeves show a much lower adhesion to metal substrates and to other coatings, when compared to thermoset materials. Consequently, they do not withstand attacks from soil stress and soil loads, common situations for buried metal pipe. Tapes and sleeves also leave a visible seam with the underlying substrate, which is susceptible to adhesive failure. Adhesive failure can provide a path through which moisture and other contaminants can migrate to undermine the integrity of the remaining bond and attack the underlying substrate.
An example of a method for repairing a coating in the field using a thermospray process is described in U.S. Pat. No. 5,792,518.